Ultrasonic echoes from deliberately launched diagnostic sound waves into tissue are attenuated in proportion to the distance that the sound waves must travel to reach the reflector, plus the distance that the resulting echoes must travel back to reach the receiver. Since sound waves are attenuated as they pass through the human body, the deeper the penetration, the greater the attenuation. Consequently, the strength of the received echoes becomes weaker with increasing depth and time. This is undesirable because it limits the ability of the system to detect the echo strength, i.e., the range over which the echoes can be heard.
To compensate for the diminishing echo strength, most medical ultrasound systems use some sort of Time Gain Compensation (TGC). Since the attenuation rate increases proportionally to the depth of the signal received, the time gain compensation must compensate for a reduced signal as the sound waves penetrate deeper into the body and are returned to the receiving transducers. TGC is a method of increasing the receiver gain as echoes are received from deeper tissues or equivalently with time. Existing TGC's are analog, since the architecture of existing medical ultrasound systems is analog. However, ultrasound imaging systems are being developed which include digital architecture.
Bit width on digital ultrasound signals can be higher than standard digital signal processing (DSP) integrated circuits support. The large bit width is needed to accommodate the large dynamic range that ultrasound signals have. Typically the signal begins large and diminishes as time passes. Commercially available hardware will not support a large bus width, only a standard bus width. Any deviation from standard size, then, is very expensive.
It would be desirable then to have a means for providing the large dynamic range needed for ultrasound signals without using a large bus width.